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Advantages and Disadvantages of Recirculating Aquaculture Systems

For commercial aquaculture, recirculating aquaculture systems have certain advantages and disadvantages over other production systems such as ponds and raceways. The main disadvantage is that they are more costly to start up and have higher operating costs (Lazur 2003). The advantages of recirculating aquaculture systems are that they:

small recirclating aquaculture system at Ohio State University South Centers (photo courtesy of Laura Tiu, Ohio State University)

One of the key factors in development of a recirculating aquaculture system is the management of water quality. Adequate filtration is critical to maintaining a healthy environment for the product being produced. Healthy water equals healthy fish. If water quality is not properly maintained, it can reduce growth and stress the animals. Stress on fish will increase time to market, can cause losses due to diseases and mortalities, and decrease feed efficiency. Because recirculating aquaculture systems are intensive, the amount of feed going into the system and wastes coming out need to be managed to maintain optimal water quality.

The management of recirculating systems relies heavily on the quantity and quality of feed and the type of filtration. Numerous filter designs are used in recirculating systems, but the overall goal of all filtration is to remove metabolic wastes, excess nutrients, and solids from the water and provide good water quality for the aquatic organisms. It is important to consider all factors when designing and investing in aquaculture systems.

The information provided on this website is meant to provide a basic introduction to recirculating aquaculture systems. An aquaculture Cooperative Extension educator can assist you in designing, developing, and maintaining specific types of aquaculture systems.

Aeration

The first limiting factor to production in recirculating aquaculture systems is dissolved oxygen (Timmons et al. 2002). Keeping fish at a high density and feeding them feeds that contain high levels of protein will reduce the amount of oxygen available in the water for fish to breathe. There are two critical reasons for aerating: 1) Adding dissolved oxygen to the system to replace that consumed by fish and the breakdown of wastes, 2) to degas other dissolved gases that, in high concentrations, can be harmful to the fish such as CO2, which can lower the pH (Timmons et al. 2002). Several methods are used to aerate systems, the most common being the use of oxygen injection. Other types of aeration systems include packed column aerators, air lifts, and air diffusers. In packed column aerators, the low oxygen water is sprayed in at the top of the column which is packed with a plastic medium, and the flow rate is kept low to keep the column from flooding. Air lifts add oxygen by injecting air into the water through a vertical pipe, agitating and circulating the water. This serves to both increase the dissolved oxygen level and degas the CO2. Air diffusers or air stones are commonly used in stagnant systems and in the home aquarium industry; however, in commercial production of fish, air stones are limited in efficiency. While fine-bubble air stones are more efficient, these types of aerators require high-pressure sources of oxygen and easily become blocked by growth of bacteria and algae (Timmons et al. 2002).

Filtration

Water quality is maintained in recirculating systems through both mechanical and biological filtration. Mechanical filtration is used to remove solids from the system. These types of filters typically include screens, granular filtration, or separators (Losordo et al. 1998). Unwanted nutrients, specifically ammonia and nitrite, are typically removed by biological filters. Having a good biological filter is critical to maintaining adequate water quality. Wastes excreted by fish are broken down through the nitrogen cycle by bacteria in the filter. How efficiently these waste products are broken down depends on the species and number of bacteria in the system. The two most common bacteria genera critical to recirculating systems are Nitrosomonas and Nitrobacter. Nitrosomonas bacteria break down the ammonia secreted by fish into nitrite. The Nitrobacter bacteria break down the nitrite to produce nitrate which is not harmful to fish until it reaches extremely high levels. Biological filters foster the growth of these bacteria by having a high surface-to-volume ratio. The high surface area provides a substrate for the bacteria to grow on. Many types of substrates are used in biological filtration including gravel, sand, plastic tubes, and plastic plates (Losordo et al. 1998).